The ZEN15 Power Switch won't have time to send any information to your hub if power goes out, but it will show as "unavailable", "offline", or a variation of this depending on your hub. Without a battery back-up, there's no way for it to send a signal without power.
Currently, the best solution for monitoring power outages is with this kit, consisting of a door/window sensor with dry contact terminals built-in, our NO/NC accessory relay, and a 12V power supply. The sensor runs on batteries and will report open/close circuits to the hub when power is lost at the relay and power supply. Once the power supply is plugged in, the sensor will also report power/no power status to the hub. If the power is cut to the supply, you can get a text or email through your Z-Wave interface notifying you. In addition to these components, you would need to make sure your hub has a back-up power source so that is can send and receive messages when the power goes down.
If you have any questions about power outage status functionality, let us know!